For an architect, looking at space is a fundamental part of what we do and in London that increasingly means looking for hidden and unrealised potential.
For a private residential dwelling, that has traditionally meant expanding to the rear, in the roof, underground and sometimes to the side. In recent years, the most advantageous of these being underground.
As a practice we have completed over 30 basement developments in 6 London boroughs providing gyms, music rooms and swimming pools as well as games rooms, home offices and additional living space.
However underground expansion is becoming increasingly difficult due to planning and neighbouring policies. And if you don’t have a London postcode, the cost of development may not deliver a return on what is likely to be a heavy investment.
The only way is up!
Building extra floors on top of existing buildings is an idea at least as old as the invention of the mansard roof. But the idea is increasingly being looked at as a way of helping solve the country’s housing crisis. With the government proposing planning rule changes to ease the addition of extra storeys, this could create thousands more homes.
How many extra homes could be delivered by building up?
Estate agent Knight Frank cross-referenced Ordnance Survey and Land Registry data to create a detailed 3D model. Its analysis concluded that rooftops in London’s fare Zones One and Two had enough space to provide 40,000 new homes. A separate study in 2016 by HTA Design for rooftop developer Apex Airspace identified the potential across Greater London at 140,000.
The government certainly seems to think the idea has legs. Earlier this month, the Ministry of Housing, Communities and Local Government proposed changes to the National Planning Policy Framework (NPPF) to make it easier to add up to two storeys to an existing building.
It also resurrected the idea that permitted development exemptions from planning applications could apply to rooftop additions.
Important lessons for architects
As a practice we have learned some valuable lessons from our roof-top project at Hamilton Court in London’s Maida Vale. Engaging early with existing residents was vital to ensure that there are benefits for them and particularly those on the top floor.
We have undertaken public engagement exercises to show residents how we can add an extra storey without compromising their living space. We have looked at additional benefits that can be provided including landscape upgrades and upgrades to communal areas including new lifts to both wings. But you have to ensure the economics stack up, which is why early engagement with a good quantity surveyor is a must.
In tight urban environments, how daylight, sunlight and overshadowing affect surrounding buildings can rule some extensions out completely. In other locations, setting the additional floors back from the edge of the existing building can help mitigate any problem.
Most rooftop extensions are constructed using prefabricated materials. Modular or panelled construction is often the best solution for speed and minimising disruption. The less man hours spent on site, the less disruption for existing residents. A prefabricated build is often lightweight, putting less stress on the existing structure and much less disruptive to the existing occupants as modules can be craned on to the top of the building. In the case of Hamilton Court, a deck was added to the roof to transfer weight before units were delivered on site.
Sympathetic cladding can also help reduce the visual impact of a rooftop development. At Hamilton Court, on the edge of a sensitive conservation area, we have taken an approach that is not apologetic but not outlandish. It is clear that the extension is a later addition, but it is sympathetic to the building below.
This is an exciting time for architects with new construction methods opening up new opportunities and I for one am happy to be spending less time underground!